Help with EGT Probe and a Suggestion on Fire Extinguisher

[Ernie Martin]

For PatrolPilot: the key question is whether the Aircraft Spruce type K probe works with the ORIGINAL factory gauge? let us know when you find out.

I understand the concern about using EGT for leaning, since you are getting an average of all cylinders and one of them may be running too lean and cause damage. That's all I've got and I use it for that (running ~ 75 degrees ROP), but I also watch other parameters like cylinder head temperature.

Ernie

[patrolpilot]

Originally Posted by Ernie Martin

For PatrolPilot: the key question is whether the Aircraft Spruce type K probe works with the ORIGINAL factory gauge? let us know when you find out.

Okay, Ernie, the short answer is yes it works.

In order to understand why it will work, I asked my mechanic at Center Line Aero (PKV) and very quickly got squared away on EGT probes. This is specifically Alcor style (Cessna, Alcor, Alcor-Tempest and generic) E & K probes. Almost all other EGT manufacturers use proprietary leads.

Alcor, one lead is shorter than the other, and leads are color-coded on the PMA FAA products.

• The short probe lead matches the long lead on the aircraft.
• The long probe lead matches the short lead on the aircraft.

Color, the PMA-FAA probes and leads match color as well.

• Type E - Black and Red
• Type K - Yellow and Red

Generic leads, know your "Type" and match leads short to long.

Okay, what types? I pulled out the parts manual when I got home, see the attached images. I hope this helps. I think I get probes now!

I wanted to point out that my image shows threaded PMA probes vs. the clamped probes. That is simply the first image that came up. Coding is exactly the same no matter how they are attached to the pipe.

[Ernie Martin]

Thank you, PatrolPilot, but I'm still VERY surprised by this. I get the lead length, but I've been under the impression that it's much more, that a gauge which uses an E probe will NOT work satisfactorily with a K probe, that you might get a reading on the gauge but perhaps not the full range of temperature readings. This may be reflected on the right of your last figure, where it says "Be certain of TYPE". When you next fly, please let me know if you seem to have good EGT readings with the K probe and the original gauges. Thanks.

Ernie

[patrolpilot]

Ernie, airplanes with a serial number 1726 or less, use an E Type. The only way it wouldn't is if a later model indicator was installed at some point in its life. In that regard, the part numbers for the "Indicator, Power Economy" are:

• Through SN 1726 - C668502-0107
• SN 1727 and up - C668503-0101

Serial numbers 1727 or greater, use a K Type. My airplane is #1795, so it uses a "K". Center Line Aero used a Alcor-Tempest 86255 from Aircraft Spruce as a replacement and yes, it responds appropriately within the range markings of the gauge. I've attached a snip from my invoice.

As a note, the indicator was an option on SN 1671 or less. It was not required equipment to identify an engine failure.

I cannot comment on the generic leads other than it should meet the resistance/ohms of the appropriate type, E or K.

[Ernie Martin]

Good to know. Thanks.

Ernie

[edasmus]

First paragraph cut and pasted from Ernie's original post:

1. I'm having trouble finding the EGT probe that is clamped on the exhaust pipe of the engine, feeding temperature to the EGT gauge. Looks as shown in the picture below. Cessna PN is C668501-0110 (the PMA version is Alcor 86156 but it's no longer made). Any info would be appreciated. Aircraft Spruce sells an MM-112 but not sure it works. Thanks.




My results:

So I flew my airplane yesterday for the first time with my Poplar Grove O/H front engine. All went very well.

I replaced the Cessna EGT probe C668501-0110 yesterday. I'm almost certain it was the original probe from Cessna and was in pretty bad shape though it was working when removed for overhaul of the front engine but it did not survive the O/H process. My rear engine had that probe replaced years ago with the above mentioned PMA version Alcor 86156 and as Ernie mentioned it is no longer available from Alcor. I spoke with them myself and they confirmed this.

This probe in question is a "Type E" probe as opposed to the more common "Type K" probes which my JPI engine monitor utilizes.

Universal (universal-co.com) makes both types of probes. The Universal MM-112 Type E probe sold by Aircraft Spruce worked in my application yesterday. There are 2 versions of the MM-112 sold. The other version is the MM-112-1. The only difference of the 2 versions is the connectors. The "-1" which is the version I used has eyelet connectors versus plug connectors on the other. I spoke to Universal (super helpful) before I purchased and they told me they were 99% certain their probe would work. The only concern was the scale may have been different. By that I mean the range the needle would display on the EGT instrument. Mine is adjustable and EGT temps being relative and specific numbers not mattering, a simple quarter turn of the adjusting knob on my instrument brought the needle movement inline with the rear engine.

So there ya go. My airplane is a 1973 C337G.

By the way, the Universal "Type K" probe works fine with my JPI monitor. I suspect it is simply a matter of making sure whether your application is a Type E or Type K probe. If you have that correct, I suspect no problems should occur!